Generally, an electrical machine, such as an electric motor or an electric generator, comprises a tubular shaped rotor and a tubular shaped stator. The stator typically comprises stator teeth which extend along a radial direction with respect to a rotary axis of the rotor. Each stator tooth ordinarily comprises circumferential surfaces which are all directed along the radial direction. The teeth are often arranged one after another along a circumferential direction. Hence, if each tooth has the same width and extends along the radial direction, the adjacent sides of adjacent teeth are not parallel with respect to each other.
For example, for a common external rotor machine, a slot between two adjacent teeth has a slot opening with a larger width than a width at the slot bottom.
FIG. 7 shows such a conventional stator 700. The conventional stator 700 comprises teeth 710, 720 which are arranged one after another along a circumferential direction. The first stator tooth 710 and the second stator tooth 720 comprise the same constant width and extends along a radial direction of the conventional stator 700. Because the width of the first and second conventional teeth 710, 720 is constant, the slots between the respective conventional teeth 710, 720 comprise different circumferential width, i.e. a slot opening comprises a larger width than the slot bottom.
As can be taken from FIG. 7, the respective coils of the windings, which generally have a rectangular shape/cross-section, can only smoothly abut with a circumferential side wall of one of the adjacent teeth 710, 720. At an opposed side, a gap 701 exists. Such gap 701 negatively affects the efficiency of an electrical machine.